The present invention relates to fluid controllers for use in vehicle steering systems, and more particularly, to such controllers which are to be used with unequal area steering cylinders.
Conventionally, many vehicles utilizing full fluid linked steering systems have included a pair of steering cylinders, arranged so that, for either direction of steering, fluid would be ported to the head end of one cylinder and to the rod end of the other cylinder, i.e., there would be the same ratio of steering movement to fluid flow for either direction of steering. Unfortunately, such an arrangement is expensive because of the two cylinders and all of the associated plumbing. On some vehicles, it is possible to use a single steering cylinder, but which is of the double rod end type, such that there is inherently the same ratio of steering movement to fluid flow for either direction of steering.
There are now a number of vehicles which utilize full fluid linked steering systems, including fluid controllers of the type to which the present invention relates, wherein it is considered desirable to utilize only one steering cylinder, having only a single rod, and thus referred to as an "unequal area" steering cylinder. An unequal area cylinder is one having only a single rod extending from the cylinder, such that the cylinder has a "rod end" having a particular cross-sectional area subject to fluid pressure ("flow area"), and a "head end" having a relatively larger cross-sectional area ("flow area"). In many vehicle applications, the use of a single, unequal area cylinder is desirable because the linkage arrangement and overall installation is simplified, and therefore, less expensive than is the case for even a double rod end cylinder.
As a separate issue, many vehicle manufacturers prefer to use fluid controllers of the type having "load reaction", a feature whereby, when the fluid controller is in neutral, the control fluid ports (i.e., the ports connected to the steering cylinder) are in fluid communication with the opposite sides of the fluid meter (typically a gerotor gear set). Thus, any external load imposed on the steered wheels will impose a reaction load on the fluid meter which, in turn, will be felt by the vehicle operator through the steering wheel.
Although load reaction is considered a very desirable feature, at least in a number of vehicle applications, there have been many occasions, prior to the present invention, in which vehicle manufacturers have been told that their steering system could not include both load reaction and a single, unequal area steering cylinder. A primary reason has been that the load reaction flow to and from the unequal area cylinder would be greater to and from the head end of the cylinder than it would be to and from the rod end of the cylinder. As a result, with the cylinder communicating to the opposite sides of the fluid meter, there would be pressure intensification occurring while the head end is contracting, and there would be cavitation occurring while the rod end is contracting. In either case, the performance of the controller would not be acceptable. While the pressure intensification could result in burst hoses, and other pressure related problems, cavitation could result in a potential loss of steering control because of voids in the fluid "column".
An additional disadvantage of the prior art load reaction and unequal area cylinder combination is that there is no position of the cylinder corresponding to a neutral position of the vehicle. The unequal area of the head end and rod end, and the balanced forces acting on the rod, mean that the pressure in the rod end is higher than that in the head end, resulting in flow from the rod end to the head end, in the absence of a steering input. This flow causes continual drift of the vehicle and rotation of the steering wheel, a condition which is not acceptable to the vehicle operator.